h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Principles of the Determination of Geologic Time - A862580
Author: Rita - U203958

Respectfully submitted for your review.

Thanks,
Rita

Nice entry!

'corrolated' should be 'correlated' and the 'editorial we' IMHO appears somewhat too often - but that's just me.

I was thinking of another method employing trees as a ruler (where the sequence of thin and thick rings is a measure of the climatic conditions prevailing when the tree was still alive) but this method has only been used to time events over the last 1000 or 2000 years, whereas you're dealing with thousands of years.

Thanks for writing this one!

Dendrochronology is utilized rather like varves to resolve yearly changes within a specific era.

Since tree trunks sometimes fossilize, the method is not limited to the Holocene but has been applied as far back as the Mesozoic, I think. However, you're correct that it isn't much good for assigning absolute dates much beyond a few thousand years. It's sometimes used to calibrate carbon 14 methods though.

Varves, the seasonal layers of sediment deposited in lakes are discernable in Precambian formations and, therefore, are somewhat more useful than dendrochronology because they can apply to eras when trees didn't yet exist.

With geologic time, we are dealing with durations of millions and sometimes billions of years, not just thousands. The average horizon resolution in this immense span of time would still be measured in hundreds of thousands of years until we approach relatively recent times.

Thanks for the suggestions.

I like the "arm-scale", gives one a good sense of proportion, fingernail, ouch. It looks a bit bitten, too.
Very informative entry. Now this is about ready to go in as is, messeems?

I liked this...a lot (ticks off another geology related entry I will never have to get round too).

One thing I did wonder, would it be worth changing the order slightly and placing the section on flora and faunal succession above the radiometric/paleomag dating methods? The methods described would then be in the order that they were first used with the recent more accurate methods towards the end.

Also how about mentioning the different elements used in radiometric and their accurate timescales (I've got this written down somewhere) a lot of people assume that carbon 14 is the most common when it is actually pretty useless for most rocks (and more useful in archaeology)...I'll see if I can dig out my notes if you like (that is literally, they're all bundled into a huge box in no order )

Anyway just a couple of thoughts, feel free to ignore them

PQ

A nice link:

Another geological timeline, not as detailed but pretty
http://www.bbc.co.uk/education/rocks/flash/indextime.html

the main page http://www.bbc.co.uk/education/rocks/ is also worth a look (not really relevent to dating but the experiments are nice)

Thanks for the suggestions, Pencil Queen. I'm not sure about the order. It wasn't exactly intended to be rigorously historical, otherwise, I would have started with Steno's Principles, but I certainly appreciate the point.

I'll see what I can find on potassium-argon and uranium-thorium-lead and so on. It might also be helpful to have a strip of paleomagnetic data to show off, what do you think?

Also, if you get a chance, review the thread Sea Change and I have been developing. He expressed the hope you could shed some light on clays in deposits associated with turpidites.

Thanks again.

I've augmented the references in the radiometric and paleomagnetic sections and also included your links under Further Reading, if that's okay. Please let me know and thanks again.

Hi Rita,

I found this entry very interesting but it is pretty obvious that you are a professional/expert/learned in the field person, and I think it would help the non-geologists amongst us if you footnoted terms such as:
igneous
tectonic spread zones
Holocene
Mesozoic
Carbon 14 methods

and so on.

I also have a couple of comments on structure which, again, might help the layperson into the 'story' more:

1. I would draw the distinction between absolute and relative dates at the beginning, as this is something that people understand. Then you can refer back to it when you introduce a new dating type.

2. i would definitely put the 'age of the earth is as long as your arm' story at the beginning, to make the reader realise immediately what kind of scale is being dealt with. Having stunned people with this information, you can then go on to say 'actually we can be pretty accurate about geologic time and here are some ways we do that'.

3. Is it worth referencing in some way (in a footnote?) the main 'eras' or whatever you call them of geologic time?

Gubernatrix

Thanks for the suggestions, Gubernatrix.

I'll provide a link to a glossary for those unfamiliar with the terminology. The initial link in the entry is a geological time scale that already lists the pertinent eras and such. Links have also been provided for radiometric methods that also explain the carbon 14 calibration process very well.

Did you review any of the links included in the main sections of entry? If you did, many of your questions or concerns would already be addressed. Consequently, I'm wondering if including additional links would be of any practical value if people don't read them. What do you think?

I think using the arm's length scale at the beginning as a hook or lead is a very insightful suggestion.

IMHO there should be as much content within the entry as is required to understand it, without clicking on links. These should be provided to point at related (or further in-depth) information. Otherwise you end up in a maze of windows or knots in your browser's history list

Brilliant entry. Don't change a thing.

The entry has been updated taking some of the criticisms into account. Thanks again for the feedback.

Unfortunately, trying to anticipate how much content is required for understanding, while trying to accommodate people who are easily confused by computer windows, seems like a pointless exercise. One ought to be able to presume a certain, average level of competence among the readers, don't you think?

When time flies like an arrow, schedules can be fatality wounded. When fruit flies like bananas, it's best not to try to swat them. I wish time flew like a rock in a creek. It might be easier to deal with all the little things that come up in a day to compete for my attention.&;D

The text dismisses dendrochronology as largely irrelevent, but it was key to callibrating the use ice core data to take climatology data back hundreds of thousands of years.

Good entry Rita, although "The Principle of Faunal and Floral Succession" could do with a few more words. Biostratigraphy was, for a while, the only accurate method of correlating disparate strata. Prior to William Smith's discovery there was only lithostratigraphy, which is riddled with pitfalls. In all, a great entry - feel free to ignore my input, I think it stands up fine as it is.

Interesting comment about ice cores. Would you mind elaborating? I don't think we're dismissing dendrochronology, just observing its constraints in establishing the overall geological timescale.

I added a little more the section, Frogbit. I'm not sure how elaborated it ought to be since it's one of the more complex concepts but see what you think.

I'm also changin the title, which seems a little awkward after some reflection.

Good stuff Rita. Though I was thinking more along the lines of "Faunal sucession (also known as biostratigraphy) was a major breakthrough in that for the first time strata in, say, Northubria could be shown to have been once connected with strata in France because they showed the same species of fossil in the same type of rock" or "Species change over time provides each strata with a 'signature' fossil which can be correlated against strata found elsewhere".
Or something. Again, please ignore. I also wrote a thing on ammonites you may(or may not) wish to link to http://www.bbc.co.uk/dna/h2g2/brunel/A703388

Anyway, good luck with this, it needs doing

Thanks, Frogbit. I've taken the liberty of linking your entry to provide a better description of Ammonites. I hope it, together with the revised section, will make the faunal succession issue somewhat clearer.

certainly I can elaborate. the dendrochronology data was used with the couple of hundred years of meteorology data to calibrate what environmental conditions suited which plant life.

this formed the basis for dendroclimatology. when used with the complete dendro data, it gave pretty comprehensive info about the climate of the planet for about 2500 years or so, and in places as far back as about 8000 years (I think).

This dendroclimatology data was then usable to calibrate the ice cores. because we have the dendro data, we have a pretty good idea of the climate. it can be veified using various seeds found in the same deposits.

as we now have ice cores dating back more than 500,000 years, we have detailed data for average planetary climate for at least that long.

This includes precice relative gas concentrations (from air bubbles trapped in the ice when it was formed, and then put through a spectrometer).

it seems to be throwing a spanner in the works for climatologists, because their models don't work properly on those timescales, and they can't figure out why.

but as comented earlier, this is a side issue.